A component used for an automobile transmission is usually subjected to casehardening treatment such as carburizing-quenching, induction hardening, or nitriding from the viewpoint of improvement in bending fatigue strength and surface fatigue strength.
Among these treatments, the “carburizing-quenching” is a treatment in which a low carbon steel is generally used, and after C has intruded and diffused in an austenite zone of a high temperature of Ac3 point or higher, quenching is performed. This treatment has an advantage of being capable of obtaining a high surface hardness and a large case depth, but has a problem of a large heat treatment distortion because this treatment is associated with transformation. Therefore, in the case where a high component accuracy is required, finishing such as grinding or honing is needed after carburizing-quenching. Also, this treatment has a problem that the fatigue strength is decreased with a so-called “nonmartensitic layer” such as a grain boundary oxidized layer or an incompletely quenched layer, which is formed on an outer layer, being a fracture starting point of bending fatigue and the like.
The “induction hardening” is a treatment in which quenching is performed by rapidly heating a steel to an austenite zone of a high temperature of Ac3 point or higher and by cooling it. This treatment has an advantage that the case depth can be regulated with relative ease, but is not a casehardening treatment in which C is intruded and diffused as in carburization. Therefore, to obtain a necessary surface hardness, case depth, and core hardness, a medium carbon steel having a C content higher than that of a steel for carburizing is generally used. However, the medium carbon steel has a problem of decreased machinability because the hardness thereof is higher than that of the low carbon steel. Also, this treatment has a problem that a high-frequency heating coil must be prepared for each component.
The “nitriding” is a treatment in which a high surface hardness and a proper case depth are obtained by intrusion and diffusion of N at a temperature of about 450 to 650° C. that is not higher than the Ac1 point. The nitriding treatment has an advantage that the heat treatment distortion is small even if a steel is, for example, oil-cooled because the treatment temperature of nitriding is lower than the treatment temperatures of carburizing-quenching and induction hardening.
Especially, of the “nitriding”, “nitrocarburizing” is a treatment in which a high surface hardness is obtained by intrusion and diffusion of N and C at a temperature of about 500 to 600° C. that is not higher than the Ac1 point. This treatment is suitable for mass production because not only the heat treatment distortion is small but also the treatment time is several hours, being shorter than that in the case where only N is intruded and diffused.
However, the conventional steel for nitriding has the problems described in the following (1) to (4).
(1) Since nitriding is a treatment in which quenching treatment from a high-temperature austenite zone is not performed, strengthening associated with martensitic transformation cannot be applied. Therefore, in order to provide a nitrified component with a desired strength, it is necessary to increase the hardness before nitriding. However, in the case where the hardness is increased by containing a large amount of alloying element, the cutting becomes difficult to perform.
(2) The aluminum chromium molybdenum steel (SACM645) specified in JIS G 4053 (2008), which is a typical steel for nitriding, can provide a high surface hardness because Cr, Al, and the like form nitrides near the surface. However, since the case depth is shallow, a high surface fatigue strength cannot be provided. Also, if the surface hardness is too high, the damage against a pair-gear becomes undesirably high.
(3) Mo (molybdenum) is an element that combines with C in steel at the nitriding temperature to form carbides, and thereby improves the core hardness after nitriding. However, since Mo is an expensive element, the use of a large amount of Mo is unfavorable in terms of economy.
(4) Also, although the heat treatment distortion of nitriding is smaller than that of carburizing quenching and induction hardening, in the case where an alloying element is contained to provide a nitrided component with a desired strength, large amounts of alloy nitrides are formed by nitriding, and the surface of the nitrided component expands. Therefore, even in nitriding, the amount of heat treatment distortion undesirably increases. In particular, an automobile ring gear poses a problem even if being subjected to slight heat treatment distortion because the automobile ring gear is nitrided after having been machined into a thin-wall final shape and having been subjected to gear cutting.
Concerning a material for nitrided component, the techniques described in, for example, Patent Documents 1 and 2 have been proposed.
Patent Document 1 discloses a “material for nitrided component excellent in broaching workability” consisting, by mass percent, of C: 0.10 to 0.40%, Si: 0.50% or less, Mn: 0.30 to 1.50%, Cr: 0.30 to 2.00%, V: more than 0.15% to 0.50%, and Al: 0.02 to 0.50%, further containing, as necessary, one element or two or more elements of Ni: 2.00% or less, Mo: 0.50% or less, S: 0.20% or less, Bi: 0.30% or less, Se: 0.30% or less, Ca: 0.10% or less, Te: 0.30% or less, Nb: 0.50% or less, and Ti: 1.00% or less, the balance of Fe and impurities, and consisting of a ferritic-pearlitic structure having a ferrite hardness of HV190 or higher, and a “method for producing nitrided component” using the material.
Patent Document 2 discloses a “material for nitrided component excellent in broaching workability” consisting, by mass percent, of C: 0.10 to 0.40%, Si: 0.50% or less, Mn: 0.30 to less than 1.50%, Cr: 0.30 to 2.00%, and Al: 0.02 to 0.50%, further containing, as necessary, one element or two or more elements of Ni: 2.00% or less, Mo: 0.50% or less, S: 0.20% or less, Bi: 0.30% or less, Se: 0.30% or less, Ca: 0.10% or less, Te: 0.30% or less, Nb: 0.50% or less, Ti: 1.00% or less, and V: 0.50% or less, the balance of Fe and impurities, and consisting of a bainitic structure having a hardness of HV210 or higher, and a “method for producing nitrided component” using the material.